As used in this application, terms such as “electrically relatively conductive” and “electrically relatively non-insulative” refer to a broad range of conductivities electrically more conductive than materials described as “electrically relatively non-conductive” and “electrically relatively insulative.” Terms such as “electrically semiconductive” refer to a broad range of conductivities between electrically relatively conductive and electrically relatively non-conductive.
Various types of electrostatically aided coating material atomizing and dispensing equipment are known. There are, for example, the devices and systems illustrated and described in U.S. Pat. Nos. 6,896,211; 5,474,236; U.S. Pat. No. 4,811,906; U.S. Pat. No. 4,369,924; published U.S. patent application 2004/0135016 A1; EP 0796663; EP 0178746; DE 4335507, and references cited in each of these.
In electrostatically-aided coating material dispensing applications for dispensing waterborne, or water-base, coating materials, the dispensing device, or atomizer, and the coating material supply are both typically held at or near ground potential. An external charging device, such as, for example, a device of the type illustrated and described in U.S. Pat. Nos. 5,085,373; 4,955,960; 4,872,616; 4,852,810; 4,771,949; 4,760,965; 4,143,819; 4,114,810; 3,408,985; 3,952,951; 3,393,662; 2,960,273; and, 2,890,388 provides an electric field through which atomized particles of the water-base, or other electrically non-insulative coating material pass between the atomizing device and the target to be coated by the atomized particles.
The grounded or approximately grounded components of the atomizer thus provide ground, or approximately ground, terminals of the electric field, one of the terminals of which is the electrodes of the indirect charging apparatus. The field contains ions through which the atomized coating material particles pass, picking up charges from the ions. The thus-charged coating material particles are attracted to grounded or near ground objects, such as the target(s) to be coated which is (are) typically conveyed past the dispensing device on a grounded or near-ground conveyor that thus also forms a terminal of the field. In a typical coating material atomization and dispensing device of this type, the ground or approximately ground condition established on components of the atomizer depends upon continuity of the non-insulative fluid stream between the atomizer and the coating material supply. However, discontinuities such as gaps and voids in the coating material stream may develop between the coating material supply source and the atomizer. Thus, fluid streams may vary in electrical conductivity and/or may not be very effective in establishing and maintaining ground paths.
In rotary atomizers with air bearing spindles, devices have been invented to bridge the gaps created by the air bearing between the stationary atomizer drive motor components and the rotating shaft and between the air bearing and its housing. However, there is typically by necessity a gap between the rotating shaft and the fluid delivery tube which typically extends through the interior of the rotating shaft. See, for example, U.S. Pat. No. 6,896,211.
Similar concerns arise in direct charging coating material dispensing systems for dispensing electrically non-conductive coating materials, such as, for example, typical volatile organic solvent-base coating materials. In such systems, the atomizer is at high magnitude, typically negative, potential. The fluid supply typically is at ground or a low magnitude potential, but grounding of the atomizer through the column of coating material extending between the coating material source and the atomizer is generally avoided because most such organic solvent-base coating materials are electrically relatively non-conductive. The coating material feed tube through which the column of coating material being dispensed is delivered to the bell cup and the inside surfaces of the bell cup are electrically relatively non-insulative and are coupled to the high magnitude potential source. Charge transfer from the atomizer to the coating material being dispensed typically occurs as the coating material flows through, and in direct contact with, the coating material feed tube and across the inside surface of the bell cup prior to atomization.